Dual-Function Radiation Shielding for Downhole Tool

ABSTRACT

A downhole tool is provided that includes radiation shielding using a material that is lighter, stronger, less expensive, and/or more commercially available than tungsten radiation shielding. Such a down-hole tool may include an electronic radiation generator that emits photons to through a source window located on an outer surface of the downhole tool. A detector may detect at least some of the photons through a detector window. Radiation shielding of the downhole tool may attenuate photons that travel from the electronic radiation generator toward the detector without passing through the detector window. The radiation shielding may include a material that has a density less than 10.5 g/cc and an element of atomic number greater than 40.

CROSS REFERENCE PARAGRAPH

This application claims the benefit of U.S. Provisional application No.63/055,914, entitled “Dual-Function Radiation Shielding for DownholeTool,” filed Jul. 24, 2020, the disclosure of which is herebyincorporated herein by reference.

BACKGROUND

This disclosure relates generally to downhole tools and, morespecifically, to radiation shielding for downhole tools that identifymaterial properties using radiation.

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present techniques,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentdisclosure. Accordingly, it should be understood that these statementsare to be read in this light, and not as an admission of any kind.

Producing hydrocarbons from a wellbore drilled into a geologicalformation is a remarkably complex endeavor. In many cases, decisionsinvolved in hydrocarbon exploration and production may be informed bymeasurements from downhole well-logging tools that are conveyed into thewellbore. The tools may be used to infer properties and characteristicsof the geological formation surrounding the wellbore and, thus, makeinformed decisions. Some such tools may include a radiation generator,to irradiate the geological formation with nuclear or photonicradiation, and sensors to make inferences from the wellbore's responseto the radiation.

In general, a detector in the downhole tool is used to detect theradiation that has interacted with the geological formation. The signalfrom the detector may be used to identify certain material properties ofthe geological formation at that depth in the wellbore, such asporosity, hydrogen content, lithology, or the like. But if radiationthat has not interacted with the geological formation strikes thedetector, that radiation may not provide useful information about thegeological formation, and thus may be considered noise. This may occur,for example, when radiation from the downhole tool sometimes passesthrough the tool directly to the detector. To prevent this fromhappening, radiation shielding material, principally tungsten and alloysthereof, may be used to attenuate this radiation. While radiationshielding may reduce the amount of radiation noise that strikes thedetector, the radiation shielding may substantially increase the size,weight, and cost of the downhole tool.

SUMMARY

A summary of certain embodiments disclosed herein is set forth below. Itshould be understood that these aspects are presented merely to providethe reader with a brief summary of these certain embodiments and thatthese aspects are not intended to limit the scope of this disclosure.Indeed, this disclosure may encompass a variety of aspects that may notbe set forth below.

For downhole tools that include an electronic radiation generator, suchas an x-ray generator, radiation shielding may be made from a lighter,stronger, less expensive, and/or more commercially available material.Thus, even though the lighter, stronger, less expensive, and/or morecommercially available material may be less effective at attenuatinghigher-energy photons emitted from radioisotopic sources such as cesium(e.g., ¹³⁷Cs), it has been discovered that the material may be quiteeffective at attenuating lower-energy photons from electronic radiationgenerators. The radiation shielding of this disclosure may have arelatively high proportion (e.g., a weight percentage of between about2.5% to 50%) of a material with a high atomic number and/or may have adensity less than about 10.5 g/cc, which is much less dense thantungsten, which has a density of greater than 17 g/cc. In this way, theradiation shielding may not only be lighter, stronger, and/or lessexpensive, but also may serve a dual function of providing mechanicalstrength to the tool. For example, the radiation shielding material maybe used in a pressure housing, an internal shield, or an externalshield.

In one example, a downhole tool has an electronic radiation generatorthat emits photons through a source window located on an outer surfaceof the downhole tool, a detector that detects the photons through adetector window, and radiation shielding, with a first material that hasa density less than 10.5 g/cc and an element of atomic number greaterthan 39, that attenuates photons that travel from the electronicradiation generator toward the detector without passing through thedetector window.

In a second example, a downhole tool has an x-ray generator thatgenerates x-rays, a radiation detector that detects a portion of thex-rays through a detector window, and radiation shielding, with ametallic alloy that contains one or more elements of atomic numbergreater than 39 and less than 75 that comprise at least 2.5 weightpercent of the metallic alloy, that serves a dual function of providingmechanical support for the downhole tool and attenuating x-rays that donot pass through the detector window.

In a third example, a method of manufacturing a downhole tool includesinstalling an x-ray generator into a pressure housing, installing anx-ray detector into the pressure housing, and installing a radiationshield between the x-ray generator and the x-ray detector where theradiation shield or the pressure housing, or both, comprise a firstmaterial that has a density less than 10.5 g/cc and comprises one ormore elements of atomic number greater than 39 at a weight percentagebetween 2.5 and 50.

Various refinements of the features noted above may be undertaken inrelation to various aspects of the present disclosure. Further featuresmay also be incorporated in these various aspects as well. Theserefinements and additional features may exist individually or in anycombination. For instance, various features discussed below in relationto one or more of the illustrated embodiments may be incorporated intoany of the above-described aspects of the present disclosure alone or inany combination. The brief summary presented above is intended only tofamiliarize the reader with certain aspects and contexts of embodimentsof the present disclosure without limitation to the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of this disclosure may be better understood upon readingthe following detailed description and upon reference to the drawings inwhich:

FIG. 1 is a depiction of a well logging operation, in accordance with anembodiment;

FIG. 2A is a schematic of an x-ray logging tool showing the paths ofescape for x-rays, in accordance with an embodiment;

FIG. 2B is a schematic of the x-ray logging tool of FIG. 2A.

FIG. 3 is a diagram of a graph comparing a spectrum of radiation from anx-ray generator with radiation from a Cesium radioisotopic source, inaccordance with an embodiment;

FIG. 4 is a graph identifying photon transmission through 1 cm ofvarious metallic alloys at 650 keV, in accordance with an embodiment;

FIG. 5 is a graph identifying photon transmission through 1 cm ofvarious metallic alloys at 100 keV, in accordance with an embodiment;

FIG. 6 is a chart identifying photon transmission through 1 cm ofvarious metallic alloys at different energy levels, in accordance withan embodiment; and

FIG. 7 is a block diagram of a method of constructing a downhole tool,in accordance with an embodiment.

DETAILED DESCRIPTION

One or more specific embodiments of the present disclosure will bedescribed below. These described embodiments are only examples of thepresently disclosed techniques. Additionally, in an effort to provide aconcise description of these embodiments, all features of an actualimplementation may not be described in the specification. It should beappreciated that in the development of any such actual implementation,as in any engineering or design project, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

When introducing elements of various embodiments of the presentdisclosure, the articles “a,” “an,” and “the” are intended to mean thatthere are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.Additionally, it should be understood that references to “oneembodiment” or “an embodiment” of the present disclosure are notintended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features.

Certain downhole tools may use an electronic radiation generator, suchas an x-ray generator, to emit radiation into a geological formation.Detectors in the downhole tools may measure the radiation once it hasinteracted with the surrounding materials. Signals from the detectorsmay be used to identify certain material properties of the geologicalformation at a particular depth in the wellbore, such as porosity,hydrogen content, lithology, or the like. If radiation that has notinteracted with the geological formation strikes a detector, however,that radiation may not provide useful information about the geologicalformation, and thus may be considered noise. This may occur, forexample, when radiation from the downhole tool sometimes passes throughthe tool directly to the detector. To prevent this from happening,radiation shielding material, such as tungsten, may be used to attenuatethis radiation. Depending on the materials used, radiation shielding mayreduce the amount of radiation noise that strikes the detector, butcould also substantially increase the size, weight, and cost of thedownhole tool.

It is now believed, however, that radiation shielding may be made ofmaterials other than tungsten. For downhole tools that include anelectronic radiation generator, such as an x-ray generator, radiationshielding may be made from a lighter, stronger, less expensive, and/ormore commercially available material. Thus, even though the lighter,stronger, less expensive, and/or more commercially available materialmay be less effective at attenuating higher-energy photons emitted fromradioisotopic sources such as cesium (e.g., ¹³⁷Cs), it has beendiscovered that the material may be quite effective at attenuatinglower-energy photons from electronic radiation generators. The radiationshielding of this disclosure may have a relatively high proportion(e.g., a weight fraction of between about 2.5% to 50%) of a materialwith an atomic number higher than 39 and lower than 75 and has or mayhave a density less than about 10.5 g/cc, which is much less dense thantungsten, which has a density of greater than 17 g/cc. In this way, theradiation shielding may not only be lighter, stronger, and/or lessexpensive, but also may serve a dual function of providing mechanicalstrength to the tool. For example, the radiation shielding material maybe used in a pressure housing, an internal shield, or an externalshield.

In this way, the radiation shielding of this disclosure may beparticular well suited to downhole tools that emit radiation from anelectronic photonic radiation generator, such as an x-ray generator,rather than a monoenergetic radioisotopic source. A monoenergeticradioisotopic source, such as cesium (e.g., ¹³⁷Cs), emits gamma rays ata single energy level. The single energy level may be relatively high(e.g., 662 keV in the case of cesium). Thus, highly effective radiationshielding made of materials such as tungsten has been used to attenuateradiation leakage through the downhole tool.

Electronic photonic radiation generators, such as x-ray generators andcertain radioisotopes, emit a spectrum of radiation of multipledifferent energy levels rather than monoenergetic radiation of a singleenergy level. Moreover, in the case of x-ray generators, the spectrum ofradiation may have lower energy than that of many monoenergeticradioisotopic sources (e.g., about 100 keV to 350 keV). Additionally,certain radioisotopes (e.g., ¹³³Ba) may emit low-energy photons similarto those emitted by an x-ray generator. Tungsten also attenuates theseenergy levels exceptionally well. Other materials (e.g., having asignificant weight percentage of atomic numbers higher than 39 and lowerthan 75, and/or densities lower than 10.5 g/cc) have been identified,however, that do not attenuate higher-energy photons (e.g., gamma-rays)as well as tungsten, but do attenuate lower-energy photons (e.g.,x-rays) such as those emitted by electronic photon generators. By atleast partially replacing some of the radiation shielding formed frommaterials such as tungsten with materials having atomic numbers lowerthan 75 that attenuate lower-energy photons, a downhole tool thatincludes an electronic radiation generator may have shielding that islighter, stronger, less expensive, and/or more available to manufacture.

With the foregoing in mind, FIG. 1 illustrates a well-logging system 10that may employ the systems and methods of this disclosure. Thewell-logging system 10 may be used to convey a downhole tool 12 througha geological formation 14 via a wellbore 16. In the example of FIG. 1 ,the downhole tool 12 is conveyed on a cable 18 via a logging winchsystem (e.g., vehicle) 20. Although the logging winch system 20 isschematically shown in FIG. 1 as a mobile logging winch system carriedby a truck, the logging winch system 20 may be substantially fixed(e.g., a long-term installation that is substantially permanent ormodular). Any suitable cable 18 for well logging may be used. The cable18 may be spooled and unspooled on a drum 22 and an auxiliary powersource 24 may provide energy to the logging winch system 20 and/or thedownhole tool 12.

Moreover, while the downhole tool 12 is described as a wireline downholetool, it should be appreciated that any suitable conveyance may be used.For example, the downhole tool 12 may instead be conveyed as alogging-while-drilling (LWD) tool as part of a bottom-hole assembly(BHA) of a drill string, conveyed on a slickline or via coiled tubing,and so forth. For the purposes of this disclosure, the downhole tool 12may be any suitable downhole tool 12 using a high-voltage power supply,for example, to generate nuclear or photonic radiation (e.g., x-rays)within the wellbore 16 (e.g., downhole environment). As discussedfurther below, the downhole tool 12 may receive energy, for example,from the auxiliary power source 24 or other store/source of sufficientelectrical energy and transform the voltage for use in producing nuclearor photonic radiation (e.g., x-rays). Further, the supplied energy maybe transformed to higher voltages within the wellbore 16, for example,via a high-voltage power supply within or proximate the downhole tool12.

Control signals 26 may be transmitted from a data processing system 28to the downhole tool 12, and data signals 26 related to the response ofthe formation 14 may be returned to the data processing system 28 fromthe downhole tool 12. The data processing system 28 may be anyelectronic data processing system 28 that can be used to carry out thesystems and methods of this disclosure. For example, the data processingsystem 28 may include a processor 30, which may execute instructionsstored in memory 32 and/or storage 34. As such, the memory 32 and/or thestorage 34 of the data processing system 28 may be any suitable articleof manufacture that can store the instructions. The memory 32 and/or thestorage 34 may be read-only memory (ROM), random-access memory (RAM),flash memory, an optical storage medium, or a hard disk drive, to name afew examples. A display 36, which may be any suitable electronicdisplay, may display images generated by the processor 30. The dataprocessing system 28 may be a local component of the logging winchsystem 20 (e.g., within the downhole tool 12), a remote device thatanalyzes data from other logging winch systems 20, a device locatedproximate to the drilling operation, or any combination thereof In someembodiments, the data processing system 28 may be a mobile computingdevice (e.g., tablet, smart phone, or laptop) or a server remote fromthe logging winch system 20.

To avoid a using a radioisotopic radiation source, an electronicradiation generator may be used. FIG. 2 is a schematic view of a portionof the downhole tool 12 that includes an x-ray generator 205. The x-raygenerator 205 may be any type of device that produces and emits photons206 by generating voltage (e.g., 150 keV, 250 keV, etc.) to form anelectric field. The source of the photons 206 produced by the x-raygenerator 205 may be at a target 207 located at one end of the x-raygenerator 205. The electric field is generated between the target 207and one or more cathodes located within the x-ray generator 205.

To determine the properties of the geological formation 14, the downholetool 12 may emit photons 206 into the geological formation 14 tointeract with the geological formation. The photons 206 are thendetected by detectors 210 located opposite the x-ray generator 205within the downhole tool 12. For example, the target 207 may emitphotons 206 as high-energy photons at an energy sufficient to cause atleast a portion of the photons 206 to scatter off elements of thegeological formation 14 (e.g., through Compton scattering) and to beabsorbed by the detectors 210. The detector 210 may include ascintillator 211 that absorbs the photons 206 and emits light based onthe energy of the absorbed photons. For example, each emission of lightmay count as a detected photon 206 (e.g., thereby adding one to a countrate of the detector 210). Further, the detector 210 may include aphotomultiplier 212 operatively coupled to the scintillator 211 todetect the light emitted by the scintillator. The measurement of theformation density is made by counting the number and energy level ofphotons 206 striking each detector 210 after being emitted by the target207 and passing through the formation.

In the illustrated embodiment, the photons 206 emitted by the target 207are directed through a source window 215 located on the outer surface ofthe downhole tool 12. While the embodiment shows a curved source window215, the source window may also be flat or any suitable shape to allowphotons 206 to pass through mostly uninhibited. The source window 215may be made out of any material such as metal, glass, or plastic.Furthermore, internal radiation shielding 217 may divide the detectors210 from the target 207 to restrict direct passage of photons 206 fromthe target 207 to the detectors 210 (e.g., internal leakage). Theinternal radiation shielding 217 may be located below the source window215 to ensure the majority of photons 206 travel through the sourcewindow 215. The internal radiation shielding 217 may be made out of ahigh density material to restrict photon 206 passage. However, radiationleakage may still occur as indicated by the photon travel paths 206.Internal radiation leakage may occur through the housing, internalradiation shielding 217, and air gaps located within the downhole tool12.

The detectors 210 may be supported by a chassis 219 within the downholetool 12. Furthermore, the chassis 219 may provide additional shieldingagainst leaked photons 206 traveling through the downhole tool 12. Thechassis 219 may surround the detectors 210 and may have openingsadjacent to the scintillator 211 and below detector windows 220. Thedetector windows 220 may be similar to the source window 215 and allowthe photons 206 to re-enter the downhole tool 12. The detector windows220 are placed in such a manner that allow a majority of the photons 206to enter the downhole tool 12 and contact the detectors 210.

In one or more embodiments, the downhole tool 12 is enclosed by apressure housing 221 that surrounds the inner components of the downholetool 12. The pressure housing 221 may secure all of the components toensure minimal movement during operation and field use of the downholetool 12. The pressure housing 221 may provide a small amount ofshielding for the interior components of the downhole tool 12 fromphotons 206. More efficient shielding may be provided by an externalshield 225 attached to the pressure housing 221 that covers a portion ofthe downhole tool 12. The external shield 225 may surround the portionof the downhole tool 12 that is exposed to the photons, such as thesurface including the source window 215 and the detector windows 220.The material of the external shield 225 may have a relatively highdensity and a high atomic number in order to scatter the photons 206 orabsorb photons that contact the external shield 225.

FIG. 2 further shows a cross section B-B of the downhole tool 12. Thedownhole tool 12 is a cylindrical design with the pressure housing 221surrounding the internal components of the downhole tool 12. As shown incross section B-B, the external shield 225 covers a portion of thedownhole tool 12. An external shield 225 may not be required to coverthe entirety of the downhole tool 12 as the photons 206 generally comein contact with only a portion of the downhole tool 12.

With the following in mind, FIG. 3 shows a graph 300 comparison of anx-ray generator spectrum and a radioisotope source. The graph 300 is aplot of an output 302 (i.e., photons leaving the x-ray generator)against energy 304 (i.e., the energy of the source) measured in keV.Cesium (e.g., ¹³⁷Cs) 306 has a single energy 304 level of 662 keV. Assuch, radiation from the Cesium 306 source initially only has Comptonscattering interactions. However, the energy 304 level for an x-raygenerator 308 ranges from about 100 keV to 350 keV. As such, photonsleaving the x-ray generator may have Compton scattering interactions andphotoelectric interactions with a scattering medium. As the photonsscatter at least once in the formation and return to the downhole tool12, the energy 304 level is reduced and the entire spectrum is moved toa lower energy with only a negligible number of photons above 200 keV.This may allow photoelectric absorption attenuation mechanisms to becomemore dominant as a photon has a greater probability of being absorbed ifthe photon energy 304 is lower. Furthermore, the photons have anincreased absorption probability if the atomic number of the scatteringmaterial is higher.

As the graph 300 shows, lower source energy changes the traditionalapproach to shielding nuclear logging tools for density and potentialenergy measurements. High density material was traditionally theradiation shielding of choice because the radioisotope source was in theCompton scattering energy range. High density materials also have a highatomic number. The density of typical pressure housing materials isslightly less than half that of tungsten alloys which reduces itsability to scatter higher energy photons. Additionally, the effectiveatomic number of typical pressure housing materials is a factor of 2 to3 times less than the effective atomic number for tungsten alloys whichreduces its ability to absorb low energy photons. Furthermore, highdensity materials may not be good candidates for pressure housingmaterials because they have lower strength, less ductility, higher cost,and poorer corrosion resistance compared to typical housing materials.

When the source energy 304 is reduced and much of the spectrum is below200 keV, as in the case of an x-ray generator 308, it is possible to useshielding materials with a higher atomic number, Z, and a density thatis not extremely high because the attenuation of the lower energyphotons is dominated by photoelectric absorption.

To further illustrate this, FIG. 4 shows a graph 400 showing thetransmission of photons through 1 cm of material at 650 keV. Graph 400relates to the use of Cesium or any other high energy radioisotope as asource for emitting photons. Graph 400 plots a list of materials basedon the photon transmission 402 through 1 cm of material against thedensity 404 of the material measured in g/cc. The materials plotted are13-8 Mo SS 406, Alloy 625 Plus 408, Hastelloy C-276 410, 17-4 SS 412,Inconel 718 414, MP35N 416, ToughMet 418, and Tungsten Alloy (Fe-Ni)420. The selection of some of the materials, such as Hastelloy C-276 410and TougMet 418, to plot on the graph 400 are due to the prominent useof these materials in typical downhole tools. Furthermore, the materials406, 408, 410, 412, 414, 416, 418, and 420 include a large proportion ofat least one alloy element (between 2.5 and 50 wt %) that has an atomicnumber above 39. As shown in this embodiment, the Tungsten Alloy (Fe-Ni)420 has a higher density than the other materials plotted on the graph400. The higher density of the Tungsten Alloy (Fe-Ni) 420 allows for theleast amount of photon transmission 402 through 1 cm of this material at650 keV at levels around 0.18. The remaining materials 406, 408, 410,412, 414, 416, and 418 have much lower densities than Tungsten Alloy(Fe-Ni) 420, but allow for much higher photon transmissions 402 through1 cm of material at 650 keV at levels are greater than 0.50. As such,there is a strong relationship between photon transmission 402 andmaterial density through a given thickness of material at a higherenergy of 650 keV where Compton scattering dominates. At high energylevels, such as 650 keV, Tungsten Alloy (Fe-Ni) 420 may be the onlysuitable material option for internal and external shielding.

FIG. 5 shows a graph 500 showing the transmission of photons through 1cm of material at 100 keV. Graph 500 relates to the use of an x-raygenerator as a source for emitting photons. Graph 500 plots a list ofmaterials based on the photon transmission 402 through 1 cm of materialagainst the density 404 of the material measured in g/cc. The materialsplotted are 13-8 Mo Stainless Steel (SS) 406, Alloy 625 Plus 408,Hastelloy C-276 410, 17-4 SS 412, Inconel 718 414, MP35N 416, ToughMet418, and Tungsten Alloy (Fe-Ni) 420. As the materials are the same asthe materials plotted in FIG. 4 , the densities 404 of the materialsremain the same. However, the photon transmission 402 through 1 cm ofeach of these materials is reduced to levels all below 0.10. In thisembodiment, Tungsten Alloy (Fe-Ni) 420 does not allow any photontransmission 402 through 1 cm of the material at 100 keV. As shown bythe graph 500, there is no direct relationship between photontransmission 402 and the density 404 of materials at low energies, suchas 100 keV. Some materials, such as Hastelloy C-276 410 and ToughMet418, may have similar densities 404, but the photon transmission 402 maydiffer by a factor close to 3 due to the difference in effective atomicnumber.

FIG. 6 further illustrates the difference in photon transmission 402 atdifferent energy levels. As shown in FIG. 6 a graph 600 shows the photontransmission 402 through materials 406, 408, 410, 412, 414, 416, and 418at 650 keV 602, 250 keV 604, and 100 keV 100. The materials are arrangedalong the graph 600 based on ascending effective atomic number of thematerial. The photon transmission 402 through the materials 406, 408,410, 412, 414, 416, and 418 decreases with decreasing energy 602, 604,and 606. As lower energy x-rays on the order of 250 keV 604 are used asthe source of radiation, photon transmission 402 through materials ismuch less than a radioisotope source (e.g., Cesium), which may have anenergy level closer to 650 keV 602. When photons with energy on theorder of 250 keV 604 (i.e., photons emitted by x-ray) scatter, theenergy will be lessened and the transmission through the materials willbe substantially reduced. At lower energy ranges, such as 100 keV 606and 250 keV 604, photoelectric effect is prominent and, thus, materialswith a higher effective atomic number have greater stopping power forphotons.

TABLE 1 Density, yield strength, and photon transmission (at differentenergy levels) for certain downhole alloys Yield ρ_(b) Strength PhotonTransmission (1 cm thick) Material [g/cc] UNS No. (ksi) 650 keV 250 keV100 keV Tungsten Alloy (Fe—Ni) 17.72 N/A 75 .18 .00035 .000000 HastelloyC-276 8.80 N10276 165 .51 .273 .002726 Inconel 625 Plus 8.36 N07716 140.53 .317 .010769 ToughMet 3AT 8.72 C72900 110 .52 .307 .007959 MP35N8.46 R30035 200 .53 .334 .019935 Inconel 718 8.09 N07718 150 .54 .349.025631 13-8Mo Stainless Steel 7.69 S13800 190 .57 .385 .052145 17-4 PHStainless Steel 7.61 S17400 150 .57 .392 .060504

Table 1 shows the typical density, yield strength, and photontransmission at different energy levels of the materials 406, 408, 410,412, 414, 416, 418, and 420 appearing in the graph 600. As can be noted,while Tungsten alloy 420 is a suitable material at a higher energyrange, such as 650 keV, the material density of Tungsten alloy is over10.5 g/cc. Thus, Tungsten alloy is not a suitable pressure housingmaterial due to the high density and low yield strength of the material.Furthermore more, the other listed materials 406, 408, 410, 412, 414,416, and 418 all have densities less than 10.5 g/cc. A material such asMP35N 416 may be a suitable pressure housing material for high pressureand high temperature applications as well as H₂S applications.Additionally, Inconel 718 414 may also be a suitable pressure housingmaterial for H₂S applications while 13-8 Mo Stainless Steel 406 may alsobe suitable for high pressure and high temperature applications.

As such, materials 406, 408, 410, 412, 414, 416, and 418 that may not beeffective at a higher energy level, such as 650 keV 602, become moreeffective for stopping photon transmission 402 at lower energy levels100 keV 606 and 250 keV 604. The materials may be used on previouslydescribed components of the downhole tool 12, such as the pressurehousing, the chassis, the internal radiation shielding, or the externalshielding. In one or more embodiments, the pressure housing may be madeof a suitable material that allows sufficient shielding of the innercomponents of the downhole tool 12. For example, the pressure housingmay be made of Hastelloy C-276 410. Indeed, the pressure housing orother components of the downhole tool 12 may include an austeniticnickel-chromium-based superalloy, a nickel-molybdenum-chromiumsuperalloy, and/or martensitic precipitation hardened stainless steel.This may allow for less material to be used on the internal radiationshielding and the external shielding as the pressure housing may providesufficient shielding to absorb photons.

Thus, the various components of a downhole tool may include these typesof materials —materials containing a relatively large proportion of atleast one alloy element (between 2.5 and 50 wt %) that has an atomicnumber above 39 and materials having a density of less than 10.5 g/cc—to effectively attenuate stray photons from an electronic radiationgenerator. These materials may be used with or without additionalshielding made from tungsten. When used in combination with tungstenshielding, the use of the materials discussed herein may permit lesstungsten shielding to be used than otherwise to achieve the same levelof photon attenuation. In other words, the downhole tool may contain asmaller amount of tungsten shielding while achieving the same orfunctionally comparable amount of photon attenuation, as compared to adownhole tool that did not also use a material that contains arelatively large proportion of at least one alloy element (between 2.5and 50 wt %) that has an atomic number above 40 and a material having adensity of less than 10.5 g/cc, as discussed above. Thus, in someembodiments, no tungsten may be used. In other embodiments, lesstungsten may be used.

FIG. 7 shows a method for constructing a downhole tool. It should beappreciated that the method of FIG. 7 is provided by way of example, andmay be performed in a different order and with more or fewer stages thandescribed here. At process block 700, an x-ray generator may beinstalled within a pressure housing. The pressure housing may fullyenclose the x-ray generator and secure the x-ray generator in place tolimit any movement from vibration or operation of the downhole tool.Furthermore, the pressure housing may include an opening and a sourcewindow to allow for photons emitted by the x-ray generator leave thedownhole tool. The pressure housing may be made of a suitable material,as discussed above, to allow for absorption of photons at an absorptionlevel that coincides with the material selected. Indeed, by selecting amaterial that contains a relatively large proportion of at least onealloy element (between 2.5 and 50 wt %) that has an atomic number above39 and a material having a density of less than 10.5 g/cc, as discussedabove, the pressure housing may serve a dual function of providingmechanical support while attenuating undesired x-rays passing through.

At process block 702, one or more detectors are installed in a chassis.The chassis may have one or more openings adjacent to the one or moredetectors to allow for the one or more detectors to receive the photonsemitted by the x-ray generator. The chassis may enclose the one or moredetectors and/or secure the one or more detectors in place to limit anymovement from vibration or operation of the downhole tool. The chassis,too, may or may not be formed using a material that contains arelatively large proportion of at least one alloy element (between 2.5and 50 wt %) that has an atomic number above 39 and a material having adensity of less than 10.5 g/cc, as discussed above. The chassis, thus,may additionally or alternatively serve a dual function of providingmechanical support while attenuating undesired x-rays passing through.In some cases, the chassis may also include internal radiation shieldingthat is formed using a material that contains a relatively largeproportion of at least one alloy element (between 2.5 and 50 wt %) thathas an atomic number above 39 and a material having a density of lessthan 10.5 g/cc, as discussed above. There may or may not be additionalradiation shielding that contains tungsten, as also discussed above.

At process block 704, the chassis is installed in the pressure housing.The chassis may be located at an opposite end of the pressure housingthan the x-ray generator. Furthermore, an internal shield may beinstalled between the chassis and the x-ray generator to further preventphoton leakage into the downhole tool. The chassis may be fully enclosedwith openings and detector windows located adjacent to the one or moredetectors.

At process block 706, an external shield is applied to the outer surfaceof the pressure housing. The external shield may enclose a portion ofthe pressure housing, in particular, the portion of the pressure housingwith the openings designed to allow the photons to enter and leave thedetection device. The external shield may be made of a suitablematerial, as described above, to allow for absorption of photons at anabsorption level that coincides with the material selected to furtherprevent photon leakage into the downhole tool. Indeed, by selecting amaterial that contains a relatively large proportion of at least onealloy element (between 2.5 and 50 wt %) that has an atomic number above39 and a material having a density of less than 10.5 g/cc, as discussedabove, the external shielding may serve a dual function of providingmechanical support while attenuating undesired x-rays passing through.

The specific embodiments described above have been shown by way ofexample, and it should be understood that these embodiments may besusceptible to various modifications and alternative forms. It should befurther understood that the claims are not intended to be limited to theparticular forms disclosed, but rather to cover all modifications,equivalents, and alternatives falling within the spirit and scope ofthis disclosure.

1. A downhole tool comprising: a radiation source configured to emitphotons; a detector configured to detect the photons that pass through adetector window; and radiation shielding configured to attenuate thephotons that travel from the radiation generator toward the detectorwithout passing through the detector window, wherein the radiationshielding comprises a metallic alloy that has a density less than 10.5g/cc and that includes at least 2.5 weight percent of an element ofatomic number greater than
 39. 2. The downhole tool of claim 1, whereinthe element of atomic number greater than 39 is at least 10 weightpercent of the metallic alloy.
 3. The downhole tool of claim 1, whereinthe element of atomic number greater than 39 is less than or equal to 50weight percent of the metallic alloy.
 4. The downhole tool of claim 1,wherein the element of atomic number greater than 39 has an atomicnumber less than
 75. 5. The downhole tool of claim 1, wherein themetallic alloy comprises an austenitic nickel-chromium-based superalloy,a nickel-molybdenum-chromium superalloy, a martensitic precipitationhardened stainless steel, or any combination thereof
 6. The downholetool of claim 1, wherein the radiation shielding comprises an internalradiation shield disposed between the electronic radiation generator andthe detector.
 7. The downhole tool of claim 6, wherein the internalradiation shield comprises tungsten.
 8. The downhole tool of claim 6,wherein the internal radiation shield does not comprise tungsten.
 9. Thedownhole tool of claim 1, wherein the radiation shielding comprises apressure housing that surrounds the radiation source and the detector.10. A downhole tool comprising: an x-ray generator configured togenerate x-rays; a radiation detector configured to detect a portion ofthe x-rays through a detector window; and radiation shielding configuredto provide mechanical support for the downhole tool and attenuate x-raysthat do not pass through the detector window, wherein the radiationshielding comprises a metallic alloy that contains an element of atomicnumber greater than 39 and less than 75 that is at least 2.5 weightpercent of the metallic alloy.
 11. The downhole tool of claim 10,wherein the metallic alloy has a density of less than 10.5 g/cc.
 12. Thedownhole tool of claim 10, wherein the radiation shielding comprises anexternal shielding on the downhole tool.
 13. The downhole tool of claim12, wherein the external shielding comprises tungsten.
 14. The downholetool of claim 12, wherein the external shielding does not comprisetungsten.
 15. The downhole tool of claim 10, wherein the radiationshielding is configured to reduce a count-rate caused by x-rays reachingthe detector other than through the detector window to less than 10% ofthe count rate that would be observed when the tool is disposed in a2.95 g/cc rock.
 16. The downhole tool of claim 10, wherein the metallicalloy comprises an austenitic nickel-chromium-based superalloy, anickel-molybdenum-chromium superalloy, a martensitic precipitationhardened stainless steel, or any combination thereof.
 17. A method ofmanufacturing a downhole tool comprising: installing an x-ray generatorinto a pressure housing; installing an x-ray detector into the pressurehousing; and installing a radiation shield between the x-ray generatorand the x-ray detector, wherein the radiation shield or the pressurehousing, or both, comprise a first material that has a density less than10.5 g/cc and comprises an element of atomic number greater than 39 at aweight percentage between 2.5 and
 50. 18. The method of claim 17,wherein the first material comprises an austenitic nickel-chromium-basedsuperalloy, a nickel-molybdenum-chromium superalloy, a martensiticprecipitation hardened stainless steel, or any combination thereof. 19.The method of claim 17, wherein installing the x-ray generator into thepressure housing and installing the x-ray detector into the pressurehousing comprises installing the x-ray generator and the x-ray detectorinto a chassis and installing the chassis into the pressure housing,wherein the chassis comprises the first material.
 20. The method ofclaim 17, comprising installing an external shield to an outer surfaceof the pressure housing, wherein the external shield comprises the firstmaterial.